Scanning Electron Microscope (SEM) systems use an electron beam in order to produce an image of the target object. During image formation, an electron beam is moving in a raster manner over the target, high energy primary electrons hit the target and cause emission of secondary electrons, those secondary electrons are collected by electron detector, amplified and processed in order to construct an image of the target.
The primary beam moving over the target (scanning) is usually implemented through application of electrical or magnetic field, which changes in time. Capacitive scanning electrodes and voltage generator are used in SEM systems with electrical scan. Scanning coils and current generator are used in SEM systems with magnetic scan.
One of the main characteristics of a SEM system is its effective data throughput or scan rate. Another important characteristics is linearity of primary beam positioning on the target during the scan line.
FIG. 1 illustrates an ideal X scan signal 20, a X scan signal 30, an ideal Y scan signal 60 and a Y scan signal 50 during a scan of three scan lines of a raster scan.
The X scan signal 30 and the Y scan signal 50 provide a “sawtooth” scan.
In FIG. 1 the x-axis represents time and the y-axis represents either X scan signal intensity or Y scan signal intensity.
X scan signal 30 includes oscillations 31. Oscillations 31 are damped during an X oscillation period Tringing1 13.
Y scan signal 50 includes oscillations 51. Y scan signal 50 has a Y scan signal amplitude B1 71.
FIG. 1 illustrates that raster scanning requires relatively fast changing X scan signal 30 and relatively slowly changing Y scan signal 50.
X scan signal 30 includes scan part 81 and retrace part 82. Scan part 81 has a duration of Tscan1 11. Retrace part 82 has a duration of Tretrace1 15. X scan signal 30 is a periodic signal having a period T1 14.
The resulting SEM image is formed during an actual scan part that has a duration of Tact1 12. Tact1 12 is smaller than Tscan1 11 by Tringing1 13.
During retrace part 82 Y scan signal 50 changes in order to move the primary beam to the next line position. The size of the change is denoted B1 71.
An analysis of X scan signal 30 shows that retrace part 82 is the most challenging part of X scan signal 30 to generate. Retrace part 82 requires the highest slew rate of the X scan signal 30, and therefore, for given X scan signal amplitude A1 41 and scan linearity, the retrace part 82 demands for widest bandwidth of the whole scan system.
The actual data throughput of such scan system is defined by scan duty cycle DC1.DC1=Tact1/Tperiod1.Tact1=T1−Tringing1−Tretrace1.
Tringing1 depends on X scan signal amplitude A1 41, Tretrace1 13 and a required scan linearity.
Therefore Tact1 depends on required scan linearity and analog performance of the given scan system, like its bandwidth and transient response.
There is a growing need to provide an efficient scanning method.